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Just to clarify my thoughts on this..
I feel that the tilde option is the worst choice. It's not an obvious underscore marker, isn't widely used (which should say enough) and I'm wondering if it wouldn't be a bad choice considering keyboard layouts.
Using the underscore characters would be second worst. The fact that it "is tradidtionally used to mark italic text whenever italics are not technically available" by authors is irrelevant in my opinion. Most users are probably unaware of that tradition and would probably find it an unintuitive marker.
My primary choice would still be the slashes.. the earlier comment that URIs should be matched on ":" is spot on and would remove any confusion with regards to URIs and italics.
--MartijnVanDerKleijn, 11 jan 2007
Shall we mark it as "rejected" somehow? Just for book keeping...
-- RadomirDopieralski, 2007 Styczeń 11 (I think that ISO dates would be saner in this context...)
Agreed, I took the liberty to put a comment box on the top of the page.
--MartijnVanDerKleijn, 2007-01-11 (many non techies are unfamiliar with ISO dates in my experience, but you're probably right)